The invention disclosed and claimed herein generally pertains to magnetic resonance (MR) angiography, i.e., to MR imaging of an artery or like vessel carrying blood or other fluid. More particularly, the invention pertains to a method of the above type wherein an amount of contrast agent, or bolus, is inserted into the vessel to enhance contrast between blood flowing through the vessel, and adjacent stationary tissue or other structure. Even more particularly, the invention pertains to a method of the above type for closely determining the arrival time of the bolus at a site or location of imaging.
It is now a well known practice in MR angiography to insert a volume of contrast agent, such as gadolinium chelate, into blood flowing along a vessel. The volume or mass of contrast agent is referred to as a bolus, and has the effect of shortening the T1 time of the blood. Thus, an MR image of the blood, acquired by a fast gradient echo or like technique, will show up very well with respect to adjacent stationary tissue of the vessel structure. These agents have been found to be very effective, particularly when used with three-dimensional (3D) MR angiographic techniques. However, if imaging occurs some minutes after the administration of contrast material, complex images are created in which distinction between target vessels (usually arterial) and other vasculature is difficult. Time-dependent leakage of contrast material into adjacent tissue increases background signal intensity, which adds a further hindrance to image interpretation. At present, there is increasing interest in imaging arteries by trying to capture first-pass arterial enhancements, resulting from use of contrast material, by coordinating the onset of a 3D MR angiographic sequence with injection of the contrast material. This approach is often referred to as "dynamic contrast material-enhanced 3D MR angiography", and aims at imaging arteries during first-pass arterial enhancement, prior to the onset of venous enhancement. Arteries targeted with this approach include the descending aorta and the mesenteric, renal, and hepatic arteries.
There are several basic approaches to capturing first-pass arterial enhancement. In the fixed transit time approach, imaging is initiated after a fixed time interval after injection. In the test bolus approach, a small test bolus of contrast is used to determine a priori the transit time of contrast from the time of injection at the injection site to the time of arrival at the imaging site. This information is then used to coordinate the initiation of a 3D MR angiographic sequence after the subsequent injection of a full bolus of contrast. In the automated trigger approach, only a full bolus of contrast is injected, and after detection of its arrival at the imaging site, a 3D MR angiographic sequence is initiated. In the latter two approaches, a method of determining the arrival of contrast at the imaging site is required.
While many studies use the fixed transit time approach, the true transit time of contrast material can vary on the order of tens of seconds from patient to patient, depending on each patient's cardiovascular status. For instance, typical transit times to the liver have been found to vary from 8 to 32 seconds. Even more important, the time window between the onsets of arterial and venous enhancement is usually just seconds in duration, and is therefore shorter than the imaging time in a typical 3D MR angiographic sequence. In the case of the liver, this time window has been noted to be as short as 8 seconds and to average approximately 16 seconds. Data collection from lower order k-space needs to occur during this time window, in order for final images to demonstrate only arterial enhancement. The shorter the time window, the more likely it becomes that the use of a fixed time delay will lead to suboptimal images that miss first-pass arterial enhancement prior to venous enhancement, and the greater the necessity for an accurate estimate of the transit time.
In applying dynamic contrast-enhanced 3D MR angiography to the carotid arteries, two important features have been noted. First, there is a very short optimal time window for imaging, typically 5-10 seconds, during which contrast material is within the arteries and the cranial circuit but has not yet reached the veins of the neck. Second, the blood-brain barrier prevents absorption of gadolinium-based contrast material, which creates a particularly strong venous signal during venous enhancement that complicates assessment of the arteries. For these two reasons, it is essential in dynamic contrast-enhanced 3D MR angiography of the carotid arteries to have accurate measurement or estimation of the patient-dependent transit time of contrast material, from injection site to imaging site.